1. Field Of The Invention
This invention is generally concerned with the use of freezing conditions in order to accomplish a degree of separation of a solvent and/or of a solute from a solute/solvent liquid solution. The herein disclosed processes have many useful ecological and industrial purposes, but they are especially well suited for: (1) converting seawater, brackish water, etc. into potable water, (2) recovering metals such as magnesium from seawater or other salt-containing bodies of water, (3) demineralizing "fresh" waters used to make carbonated beverages and (4) recovering solutes and/or solvents from industrial solute/solvent solutions.
Numerous methods and/or systems have been suggested and/or developed for employing freezing conditions to produce potable water or other "pure" solvents or solutes from solute/solvent solutions. Most of these processes have not, however, experienced wholehearted economic and/or technical acceptance. For example, one of the principal problems inherent in most desalination processes which employ freezing conditions as their modus operandi is their inability to produce potable water at economically acceptable costs. That is to say that not only must a desalination process be able to produce large volumes of potable water, it also must be able to do so at locally acceptable costs. Cost considerations are particularly important to this art because many areas of the world having the most acute potable water shortages also are characterized by their very low income levels.
In many processes employing freezing conditions to produce potable water, seawater is simply frozen to produce an ice product and/or flash-frozen to produce a water vapor product which is recondensed to obtain pure water. Both of these freezing techniques require a great deal of expensive refrigeration and/or compressor capacity. The flash freezing techniques are especially expensive because they employ vacuum/freezer apparatus wherein a vacuum is created in order to evaporate large volumes of water in the form of water vapor. Such evaporation can only be accomplished by the expenditure of a great deal of mechanical work. It then must be condensed back to water by another large expenditure of mechanical work. This all goes to say that the refrigeration, vacuum creation and condensation steps of such processes require considerable amounts of mechanical and/or electrical energy which can only be obtained at high fuel costs and/or high capital equipment costs. Consequently, freezing and flash-freezing processes to produce potable water or other "pure" liquid solvents have not been heretofore widely employed.
2. Description of the Prior Art
Some representative freezing techniques heretofore suggested or used for the production of potable water from seawater are taught in the following patent references. They are more or less representative of this entire art. For example, U.S. Pat. No. 4,236,382 teaches a desalination process wherein a deaerated, ice water solution is first flash vaporized under a highly reduced pressure to form a low pressure water vapor brine and ice crystals. The ice is cleansed of surface brine in a counter-washer and then melted inside heat conductive conduits. Low pressure water vapor is also desublimed to form a desublimate (ice) on the outside of conduits which are specially designed for this purpose. This particular process also employs the latent heat of desublimation of the desublimate in order to supply the heat needed in the ice making portion of the process.
U.S. Pat. No. 3,664,145 teaches a method for separating a solvent (e.g., water) in substantially pure form from a solution (e.g., seawater) wherein a vacuum freezer is employed in order to produce vapors and a slurry of solvent and solute. The product materials are separated by various mechanical steps which form a large part of this particular patent disclosure.
U.S. Pat. No. 3,070,969 teaches a process which also employs vacuum freezing conditions to separate dissolved salts, such as those found in seawater, from a solvent solution. The seawater is vacuum frozen by use of a novel arrangement of equipment components in order to both concentrate solute salts contained in the liquid component of the solution and in order to collect the solvent component of the solution in the form of a frozen solvent material (e.g., ice).
U.S. Pat. No. 3,214,371 teaches a desalination process which is based upon formation of large ice crystals in brine through the use of certain water clathrate substances such as propane hydrate. The ice crystals are separated from the brine (and from the clathrate substance) and then melted in a process employing the latent heat absorbing capacity of the ice to further promote formation of a hydrate produced from a brine and water clathrate feedstock.
U.S. Pat. No. 4,505,728 teaches a vacuum freezing, multiple phase transformation process employing flash vaporization in a vacuum freezing zone in order to produce water vapor which is then condensed to water.
U.S. Pat. No. 3,213,633 teaches a freeze concentration process employing a refrigerant vapor in order to freeze water. This is accomplished by direct contact of the water with a totally vaporizable liquid refrigerant.
European Patent 78-164 teaches a purification process based upon flash evaporation of a solvent from a solution at a pressure below the triple point of the solvent.
Again, the principal drawback with most of the above noted freezing or vacuum freezing methods is that the amount of pure solvent which can be produced by such methods is directly proportioned to the size and efficiency of the mechanical compressor(s) needed to produce the refrigeration and/or vacuum freeze conditions necessary to carry out such processes. Consequently, the extremely high costs of vacuum freeze processes such as these usually can only be justified in processes wherein the value of the end product (e.g., blood plasma and freeze-dried coffee) is very high. Such processes are not, however, normally used to produce potable water, not because they do not work, but rather because they are prohibitively expensive to build, maintain and operate in the context of a product (e.g., water) of relative low economic value.
In response to all of these technical problems and economic circumstances, the herein disclosed processes are intended to provide methods, systems and apparatus for separating a solvent (e.g., potable water), in a highly refined or even substantially pure form, from a solute/solvent solution (e.g., seawater) at substantially lower costs than those which can be obtained by conventional refrigeration and/or flash freezing methods. These processes also can be equally well employed to obtain a solute product (e.g., a salt) from a solute/solvent solution (e.g., seawater).
The herein disclosed three phase separation processes do not require any compressor equipment; or, if compressor equipment is in fact employed, it need not have a great deal of compressor capacity relative to that required by the above-noted prior art flash vaporization processes. It should also be noted that small, conventional, mild steel, equipment such as piping, low pressure vessels and heat exchangers will normally suffice for use in the processes described in this patent disclosure. Moreover, the labor and maintenance requirements of applicant's processes are much less than those generally associated with flash-freezing processes.